The field of the invention is electronic controls for motors, including brushless DC motors, synchronous ac motors and switched reluctance motors.
Permanent magnet motors are excellent candidates for traction drives in electric/hybrid-electric vehicle applications because of their higher efficiency. Due to a long effective air gap, PM motors tend to have low inductance. Recent design techniques for high power PM motors (rated at several tens of kWs) for electric vehicle and hybrid electrical vehicle propulsion have used an iron-less stator structure to eliminate stator iron loss. This design also reduces stator inductance well below 100 xcexcH. While these types of very low inductance PM motors have the advantages of fast current control response and a linear relationship between current and its developed torque, they impose stringent current regulation demands for the inverter to obtain acceptable current ripple, which should be typically below 5%.
A PM motor can be excited in an ac synchronous mode or brushless dc (BLDC) mode using a dc link inverter to control three-phase switching of current in the windings of a 3-phase motor. The latter excitation provides a low cost drive system, which is well suited for PM motors having a trapezoidal back EMF. There is, however, in the known inverter technology, a problem of unacceptable current ripple with low inductance PM motors.
The maximum current ripple is inversely proportional to the motor inductance and the inverter switching frequency. A standard inverter with the most commonly used IGBT switching devices will produce an unacceptable maximum current ripple of 25% due to the IGBT""s switching frequency, which is limited to 20 kHz. This high current ripple not only causes additional motor losses but also requires that the inverter be rated higher to handle the high peak current.
For a given switching frequency, the current ripple can be reduced by adding an external inductor in each phase to increase the inductance. However, inductors rated for high current rating are bulky and this produces a large inductance which undesirably slows current control response. Another method of reducing current ripple uses a step-down chopper to regulate the dc voltage applied to the inverter. This also requires an additional inductor. These approaches have not resulted in a control with the desired features of operation and construction.
In order to reduce current ripple and use permanent magnet (PM) motors having very low inductance, the invention provides a multilevel voltage source for the dc source of a multiple phase bridge inverter. The multilevel voltage source has multiple voltage-controlled cells cascaded to provide the necessary voltage to the motor. To regulate motor current with low ripple, the output voltage of the cells is adjusted through dc voltage level stepping and pulse width modulation (PWM) depending on the amplitude of the motor back electromotive force (EMF).
It is a primary object of the invention to reduce current ripple in a PM motor powered by a dc link inverter to below 5% over the full speed range of the motor.
It is a further object of the invention to provide controls for brushless DC motors, AC synchronous motors and switched reluctance motors having low inductance.
Other objects and advantages of the invention, besides those discussed above, will be apparent to those of ordinary skill in the art from the description of the preferred embodiments which follows. In the description reference is made to the accompanying drawings, which form a part hereof, and which illustrate examples of the invention. Such examples, however are not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.